The continued trend in the electronics industry, as technology advances, is toward increased circuit board densities motivated by the need for smaller, thinner integrated circuit (IC) packages. As a result, the fabrication of these IC packages becomes increasingly difficult due to the decreased spacing between adjacent circuits. The presence or generation of undesirable contamination particles (such as dust particles or moisture droplets as small as 0.10 micrometers and above) during the manufacturing and processing of integrated circuits can often cause physical defects or other quality control problems. This is especially problemsome in semiconductor manufacture, for example, where semiconductor wafer processing geometries can approach 0.1 micrometer and below with line widths of 0.35 microns.
In the disk drive industry, disk drive platters have continued to diminish in size while at the same time offered increased storage capacity. These advancements are due in part to substantially increased track densities. Similar to IC fabrication, undesirable contamination particles during the manufacturing and processing of these disks pose significant physical defect or other quality control problems.
Accordingly, typical processing environments for semiconductors, disk drives or other particle contamination sensitive electronic components utilize standardized "clean room" techniques. Depending upon the cleanroom classification, these rooms primarily rely on filtering techniques, as well as other devices, to continuously remove particles having geometries of about 0.10 micrometers or larger. More recently, standardized mechanical interface (SMIF) systems have been integrated into the fabrication techniques of integrated circuits, disk drives and other electronic components to further reduce particle contamination which may be introduced during storage, transport or transfer between processing steps. By mechanically minimizing the volume of gaseous media surrounding the semiconductor wafers during fabrication, the SMIF system insures that much of this gas is essentially stationary relative the wafers, and that the contaminant particles from the exterior "ambient" environment cannot enter the wafer environment.
While SMIF systems have substantially reduced particle contamination, and thus, improved semiconductor or disk drive fabrication, these components often still require manual manipulation at some time between fabrication processes. As is generally the case, different processes associated with a particular fabrication machine require different processing times. Moreover, when problems develop along one stage of the fabrication process, the whole or part of the fabrication process may be shut down. The electronic components which may be at differing stages of the fabrication process, accordingly, will likely require temporary storage in cassette-type devices until further processing can be continued. Unfortunately, it is extremely difficult to maintain these large clean room environments virtually free of 0.1 micrometer sized particles or less due to the constant presence of people and operating machinery within the clean room. Contamination particles eventually come to rest upon the storage cassettes which may become dislodged upon handling thereof.
To address this problem, mobile storage systems have thus been developed to temporarily store electronic components therein which a re sensitive to particle contamination during manufacture and processing. These units are essentially mini-cleanroom storage facilities designed to store and transport a plurality of these cassettes which support the electronic components therein. Since these mobile storage systems are self-contained, they may be employed either internally in the cleanroom from one fabrication station to the next, or externally from cleanroom-to-cleanroom.
Typically, these portable facilities provide a storage cabinet having a rectangular-shaped cavity wherein a plurality of vertically-spaced stainless steel wire shelves are provided extending horizontally across the cavity therein. A blower assembly is positioned on top of the storage cabinet which generates a downdraft flowing through the cavity from an upper portion of the cavity to a bottom portion thereof. Positioned between the blower assembly and the upper portion of the cavity is a filter device extending transversely across the cavity so that the airflow generated by the blower assembly passes through the filter before flowing through the cavity. The filtered air is then vented out of the bottom of the cavity through a screen device and into the surrounding environment. One such device typical in this field is the PUREFLOW.TM. mobile storage system by Terra Universal Inc.
While these mobile storage systems perform satisfactorily under most circumstances, several problems are inherent with this design. For one, it is highly desirable to prolong the life of the filter elements due to the costs associated with replacement. Typically, the HEPA or ULPA filters employed in these systems are more than $200.00 to replace. Depending upon the cleanroom classification which the storage system resides (i.e., from class 100 to class 1) the life of the filter can vary drastically. In the worst case scenario where the mobile storage unit is transferred between cleanrooms, the filter element may require replacement at least once every couple of months or in some cases even weeks.
Another problem associated with this design is that due to the top mounted blower assembly, the relative height of these units is increased which reduces the ease of maneuverability and storage. Moreover, the weight of the upper mounted blower assembly together with the overall height increase raises the center of gravity of the unit. As a result, the stability of the mobile storage system is further compromised.
Moreover, these flow-through open systems are difficult to pressurize since the system must be choked to generate an adequate pressurization. Thus, these designs require more blower power to increase pressurization of the storage cavity. As a result, the battery run time is substantially reduced during true mobile operation.
Finally, these units are very noisy to operate due to the flow through design. The lack of baffling enables the blower noise to be directly emitted from the bottom of the unit. This is especially problemsome in the closed confines of smaller cleanrooms.